Along with safety driving measures of cars, pleasant driving also has been one of the current issues. Among such problems there has been raised the problem of an offensive noise that is produced within the cabin of the car when air is drawn into the nose of an air cleaner in the engine compartment. It has been determined that the indoor acoustic characteristics of a car, for example, the acoustic characteristics in the right and left directions, is constant, and remarkably affected by the primary and secondary components of explosion as the secondary component of the revolution of the engine.
The influence of the abovementioned first component has been reduced to some extent nowadays by elongating the air cleaner nose and making its section smaller.
On the other hand, in order to cope with the exhaust gas regulation as one of counter-measures of environmental pollution due to the cars, there has been employed a cool air intake system which includes, as shown in FIG. 1, a cool air intake nose 6 of which base portion is fastened to the nose 3 of the air cleaner 2 inside the engine compartment 1 and interposed between said nose 3 and the front frame 5 in the proximity of the head lamp 4. This exhibits a supplementary effect on the abovementioned intake noise.
The provision of this cool air intake nose 6 provides a sufficient effect as shown by curves in FIG. 2 wherein the number of revolutions N of the engine (the aforementioned explosion primary component) and the noise level S are respectively plotted on the abscissa and the ordinate, and the full line representing the use of the cool air intake nose 6 clearly demonstrates drastic increase in the noise in comparison with the dotted line representing the non-use of the same.
However, the size of the engine room 1 of a car is predetermined in accordance with the model of the car and the space for extending the cool air intake nose is therefore limited substantially to a predetermined distance because the engine room is stuffed with a battery, lamps and other associated instruments and components, to say nothing of the engine itself.
Consequently, air column resonance of a half wavelength is freshly generated in the cool air intake nose 6 as illustrated in FIG. 3, and matches with the aforementioned indoor acoustic characteristics and produces a confined noise inside the cabin.
As mentioned above, however, there is hardly any room inside the engine room for changing the length, size and position of the cool air intake nose 6.
It has been confirmed that the problem of noise amplification from the cool air intake nose occurs especially in cars having a four-cylinder engine and the like, and is particularly acute when the number of revolutions of the engine reaches 3,000 r.p.m., for example.